The objective of the study is to determine if either atracurium or vecuronium is better for muscle relaxation in critically ill children requiring mechanical ventilation. The endpoints of the study are the number of dose adjustments for each agent during the course of therapy and the time to return of normal neuro-muscular transmission after the agents are discontinued. Two patients have been enrolled into the study to date. Since the study is blinded no results are available. Although neuromuscular blocking agents have been used as adjuncts in critically ill patients on mechanical ventilation for the past 30 years, there has never been a controlled trial evaluating their use. This is the first scientific trial evaluating the dosing requirements and recovery times of these agents in critically ill patients. Two of 20 patients have been enrolled to date. The remaining 18 patients should be enrolled by the spring of 1994.